The following Mole Calculation question is not difficult but many O Levels Chemistry students tends to make mistake in exams, where time is not a luxury. When the same question was posed to my regular classes of Sec 3 and 4 O Levels and IP students, i realised that many fall into the trap also.
Let’s take a look at the question.
Question:
The relative atomic mass of chlorine is 35.5. What is the mass of 2 mol of chlorine gas?
A. 142g
B. 71g
C. 35.5g
D. 2g
Before we continue, let’s do a quick recap on the formula we need to use to solve this question, which is:
Mass = Mole x Mr of Molecule
Try it first on your own! Once you have an answer, find out the Correct Answer, Common Mistakes made by Students as well as my Suggested Solution below.
Enjoy!
Suggested Answer:
A
Common Mistakes made by students:
Group of students will make the mistake by choosing answer as B. This is because they didn’t realised that Chlorine Gas is a Diatomic Gas and has the chemical formula as Cl2.
Suggested Solution:
If you are aware that Chlorine Gas is Cl2, then the working would look like this:
Mr of Cl2 = 2 x 35.5 = 71
Mass = Mole x Mr of Cl2 = 2 x 71 = 142g
Hope you learn something here today.
Stay tune to the next post. =)
PS: Leave me a comment. I would love to hear from you.
Related Articles:
- O Level Chemistry: Common Errors in Calculating Empirical Formula
- O Level Chemistry Essential Concepts Revision Workshop – Post Analysis Series – Part 1
- O Level Chemistry: Determining Molecular Formula from Empirical Formula
- Chemistry Examination Tips – Paper 2
- O Level Chemistry: Mole Concepts / Chemical Calculations
What is the mass of CaO when 5g of CaCO3 is burnt….can you help me please??
Reply
BS Reply:
May 13th, 2011 at 4:01 pm
Decomposition takes place.
CaCO3-> CaO + CO2
Moles of CaCO3= 5g X (20+6+8(3))
= 250
Hence, CaCO3 : CaO
1 : 1
Therefore, Moles of CaO= 250
Mass of CaO= 250 X (20 +
= 7000g
Reply
Maisie Reply:
June 16th, 2011 at 8:00 pm
Hi,
I thought
Moles of Caco3= Mass/ Mr
= 5/[40+12+(3x16)]
=0.05 moles?
Reply
thank u but if you an post question from chemistry book (matters) then it would be really hepful for me. thank u very much again.
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This question has certainly proven useful. Initially, I made the mistakes that most students would. Now I know that I have do more revising and practices
Thank you.
Reply
oh myy gosh this has definately helped me thnx
Reply
If element M of group I combine with element X of group VI ,the formula of the compound formed is?please i need a solution and an answer!
Reply
BS Reply:
June 21st, 2011 at 11:06 am
The charge of element M would be +1(Group 1) while the charge of element X would be -1(Group 7). Hence, the compound formed would be MX(ionic compound).
Reply